ci: cancel Build all runs of closed pull requests (#16380)

.github/workflows/build_all.yml

@@ -57,7 +57,9 @@ on:
         default: false
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.event_name }}-${{ github.event.pull_request.number || github.ref }}
+  # cancel_closed_pr_builds.yml joins this group by name, so update that file
+  # with any change here.
+  group: build-all-${{ github.event_name }}-${{ github.event.pull_request.number || github.ref }}
   # Pushes don't cancel a running build, because only a finished branch build
   # saves caches every pull request can restore.
   cancel-in-progress: ${{ github.event_name == 'pull_request' }}

.github/workflows/cancel_closed_pr_builds.yml

@@ -0,0 +1,21 @@
+name: Cancel builds of closed pull requests
+
+on:
+  # pull_request_target needs no approval for first-time contributors and must never
+  # check out pull request code.
+  pull_request_target:
+    types:
+      - closed
+
+permissions: {}
+
+jobs:
+  cancel:
+    # Matches the concurrency group in build_all.yml, so joining it cancels the pull
+    # request's unfinished Build all run.
+    concurrency:
+      group: build-all-pull_request-${{ github.event.pull_request.number }}
+      cancel-in-progress: true
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo "Joined concurrency group build-all-pull_request-${{ github.event.pull_request.number }}."